The present invention relates to an optical disc and an optical pickup control system for reproducing the optical disc, and more particularly to a system for reproducing information on an optical disc having a high recording density.
The optical disc has a central hole and a recording track spirally formed on the outer periphery of the hole for recording information thereon.
FIG. 18a shows a structure of a conventional optical disc.
An optical disc 60 comprises a substrate 61 made of a transparent material of acrylic resin such as polymethylmethacrylate (PMMA) resin. On the substrate 61, a plurality of phase pits 62 as recorded information are formed. A reflection film 65 is coated on the substrate 61. The reflection film 65 is made of metal such as aluminum coated by vacuum deposition. A protection film 64 made of plastic is provided for covering the reflection film 65.
The information recorded on the disc 60 is read out by a laser beam 70. A spot diameter r of the beam 70 is represented by an equation as follow. EQU r=k.multidot..lambda./NA
where k is constant (about 0.82, if the spot diameter of the beam is determined in the range between a peak intensity of the spot and a light intensity of 1/e.sup.2), .lambda. is the wavelength of the laser beam, and NA is Numerical Aperture (normally 0.4 to 0.6 for reproduction).
The phase pits 62 are irradiated with the laser beam 70 through the substrate 61 for reproducing the information. As shown in FIG. 18b, the light 71 reflected at the area other than the pit 62 is detected to be a high intensity. The light 71' reflected on the pit 62 is detected to be low of the intensity because of diffusion.
The information is reproduced in accordance with the quantity of the reflected light. The spot diameter of the beam is determined based on the wavelength of the laser beam. In the high-density disc, in order to increase the recording density, the pitch of the recording track is reduced or the length of the pit is decreased. Accordingly, as shown in FIG. 18c, a plurality of phase pits 80a, 80b and 81 are included in a beam spot 82. Information on the pits 80a, 80b, 81 are detected at the same time. Since the information can not be separated with each other, it is impossible to reproduce information with accuracy.